With the high performance and high definition of a liquid crystal display, as a polarizing plate protective film used for a polarizing plate, the polarizing plate protective film is required to have a performance durable to a severe environmental variation in terms of the dimensional stability of a polarizing plate, the curl characteristics of a polarizing plate, and the surface nature of a polarizing plate.
As a polarizing plate protective film of the liquid crystal display used generally at present, a film whose material is cellulose is used. As for a cellulose ester film, generally, a solution casting film forming method is taken from viewpoints of securement of flatness. Therefore, in the solution casting process, since a film expands to a thickness direction with evaporation of a solvent, the refractive index of the thickness direction becomes lower than that in a film plane. As a result of consideration for a display mode of a liquid display device, other retardation film and retardation of members, a polarizer protective film suitable for securing a view angle of a liquid crystal device is required to be a film in which the refractive index in the thickness direction is equal to that in the in-plane direction, or to be a polarizing plate protective film in which the in-plane refractive index is higher.
In Patent document 1, it is disclosed that by the addition of ethylenic polymer, the deterioration of a polarizer under high temperature and high humidity becomes small and a film in which retardation in the thickness direction is small can be obtained. However, it turned out that only with the addition of ethylenic polymer, it is difficult to obtain a currently-required film in which retardation in the thickness direction is small and a polarizing plate excellent in the curl characteristics of a polarizing plate and the surface nature of a polarizing plate. Further, it turned out that the more the ethylenic polymer is added in order to reduce retardation, the more the above-mentioned characteristics deteriorate.
On the other hand, in a liquid crystal display, in order to make it easy to observe a displayed image, an illumination panel is arranged behind a liquid crystal display panel, and light from the illumination panel is transmitted from the rear side to the front side of the liquid crystal display panel so as to display an image.
The backlight used here is required to irradiate the whole liquid crystal display screen uniformly. Recently, in a thin type liquid crystal display of a note book type personal computer required to be miniature and thin more, a sidelight type plane light source to make light to enter a liquid crystal displace screen from its side is used preferably. Generally, in this sidelight type plane light source, adopted is a light guiding plate system to use a light guiding plate capable of transmitting and dispersing light uniformly and to irradiate the whole liquid crystal display screen uniformly. This light guiding plate system comprises a light guiding plate, a light source arranged at a side where light of both sides of the light guiding plate enters, a reflecting plate arranged at the back side of the light guiding plate, a light diffusing film to scatter and disperse light coming out from a light emitting surface being a front surface of the light guiding plate and to make the luminance of an irradiation surface uniform.
A milky white resin film described in Patent document 2 is used for a light diffusing film applied to such an illumination panel. It makes light emitted from a light guiding plate to diffuse light, makes light from a light source uniform, and spreads light, whereby it contributes to improve the view angle of a display device.
However, the light diffusing film is required to be excellent more in optical isotropy, high flatness, smooth surface, little curl, and good plane quality. Further, a producing method for a light diffusing film with high productivity is requested.
Moreover, although various light diffusing films are proposed by, for example, Patent documents 3 to Patent document 8, these proposed light diffusing films have a complicated structure and is not sufficient as a light diffusing film having good optical characteristics and plane quality.
Patent document 1: Japanese Patent Unexamined Publication No. 2003-12859, Official gazette
Patent document 2: Japanese Patent Unexamined Publication No. 2001-172403, Official gazette
Patent document 3: Japanese Patent Unexamined Publication No. 11-6905, Official gazette
Patent document 4: Japanese Patent Unexamined Publication No. 11-64611, Official gazette
Patent document 5: Japanese Patent Unexamined Publication No. 10-142406, Official gazette
Patent document 6: Japanese Patent Unexamined Publication No. 9-159837, Official gazette
Patent document 7: Japanese Patent Unexamined Publication No. 7-13002, Official gazette
Patent document 8: Japanese Patent Unexamined Publication No. 11-271510, Official gazette